100 



drawing the points d, a and a, d, together we get figure 59, which is m 

 general plan, a section across one of the ambulacra of a Pentremite. 

 On examining nearly all the published figures of species of this genus I 

 find that there is a series of forms which exhibit a gradual passage, from 

 those with the hydrospires almost entirely exposed, as in fig. 56, through 

 others in which they are crowded more and more under the arms, until afc 

 length they become altogether internal. 



Fig. 56 Fig. 57 Fig. 58 



Fig. 56. Summit of C. acutus McCoy, w, mouth and vent ; d, d, suture across the posterior 

 hydrospire. 57. Section across the hydrospire from d, to d, at a, is the place of the- 

 arm. 59 . Section contracted as in fig. 58. . Summit of Pentremites caryophyllatus 

 De Koninck. 



In C. acutus, fig. 56, only a small portion of the hydrospire is concealed 

 under the arm. In C. Canadensis, a new species lately discovered in 

 the shales of the Hamilton group in Canada West, each of the four inter- 

 radial spaces, in which the hydrospires are placed, is excavated, in such a 

 manner as to form a small triangidar pyramid, with two of its faces slop- 

 ing down toward the sides of the two adjacent arms. On these two slopes 

 are placed the hydrospires, which appear to have one fissure entirely 

 under, and another partly under the arm, five others being fully exposed. 

 S. S. Lyon has described a species under the name of (7. alternatus in. 

 the Geology of Kentucky,' 'vol. iii., p. 491, from the Devonian rocks of 

 that State, which closely resembles C, Canadensis, but is still distinct 

 thereform. Speaking of the structure of the summit he says ; '* the 

 depressed triangular intervening spaces are filled with seven or more thin 

 pieces, lying parallel to the pseudambulacral fields, articulating with the 

 summit of the second radial, and the prominent ridge lying between the 

 pseudambulacrae. These pieces were evidently capable of being com- 

 pressed or depressed : the * point ' at the lateral junction of the second 

 radials is in some specimens folded over toward the mouth so as to entirely 

 obscure these triangular spaces by covering them.'^ This important 

 observation proves that even in the same species the hydrospires may be 

 either partly or wholly concealed under the arm. The " point " to which; 



